The invention pertains to stents and is particularly adapted to stents made from bioabsorbable materials.
Self-expanding stents, such as braided or woven stents, for surgical implantation in body lumens (tubular vessels) are known for repairing or strengthening the vessels. A stent essentially is a hollow tube that may take the place or at least supplement the body vessel. With respect to the medical condition of stenosis, in which a body lumen tends to collapse or otherwise close, the stent supports the wall of the vessel to prevent it from collapsing or closing. A blood vessel that is narrowed due to the build up of intra-vascular plaque is one example of a stenosis. With respect to the medical condition of aneurism, in which a body lumen is weakened and cannot properly withstand the internal pressure within the vessel and bulges out or ruptures, the stent serves essentially the opposite function in that it substitutes for or supplements a weakened portion of the vessel. Stents are known for insertion in blood vessels, trachea, esophagus, urethra, ureter, nasal passages, ductal systems, etc.
Many different types of stents are commercially available at this time. Most stents need to be radially constricted, i.e., reduced in diameter, in order that they can be more easily inserted into the body lumen. Once they are in situ, the stent can be radially expanded to the desired diameter. Stents are known that are fabricated from rigid, but flexible materials that, when bent by force tend to retain the bent shape. Such stents may be inserted into the body lumen in an unstressed radially minimal shape while mounted over a deflated balloon. When the stent is in situ, the balloon is inflated in order to radially expand the stent, which will then retain the radially expanded shape after the balloon is deflated and removed.
Another type of stent is termed a self-expanding stent. Self-expanding stents may be woven in a variety of single or multiple strand woven designs which can be compressed radially, but will expand to its original shape once the constrictive force is removed. These woven designs are often made of shape memory materials, such as Nitinol, that expands when subjected to body temperature.
Another type of self-expanding stent is disclosed, for instance, in U.S. Pat. No. 1,205,743, issued to Didcott and incorporated herein by reference. Didcott discloses a braided, surgical dilator stent particularly adapted for esophageal dilation, but which can be adapted for use in other body vessels. This patent discloses a stent comprising a hollow tubular member the wall of which is formed of a series of individual flexible thread elements, each of which extends helically around the central longitudinal axis of the stent. A number of the flexible thread elements have the same direction of winding and are displaced relative to each other about the cylindrical surface of the stent. They cross a second plurality of helical thread elements which are also displaced relative to each other about the cylindrical surface of the stent, but having the opposite direction of winding. Accordingly, as shown in FIG. 1, the threads 12 of the first set of threads cross the threads 14 of the second set of threads at crossing points 16. FIG. 1 illustrates an embodiment in which the crossing threads are fully interlaced, however, it should be understood that the crossing threads may be interlaced at other frequencies, e.g., every other crossing point or every third crossing point.
As the stent is axially stretched, i.e., the longitudinal ends 18 and 20 are forced away from each other, the diameter reduces. Likewise, if the wall of the stent is constricted so as to reduce the stent""s diameter, the stent elongates. In other words, radial constriction and axial elongation go hand in hand. When the force is released, the stent tends to spring back to its original diameter and length. The force with which the stent returns to its original state depends on many factors including the rigidity of the individual threads, the number of threads, and the original (resting) crossing angle a of the threads. The rigidity of the threads, in turn, depends upon such factors as the material out of which they are fabricated and the thickness of the threads. In general, the greater the rigidity and/or the greater the resting crossing angle of the threads, the greater the radial expansion force.
The desirable radial expansion force for a given stent depends on the application. When used in blood vessels, stents are commonly used to treat stenosis and particularly to hold the vessel open when it has become narrowed by either internal or external forces. Accordingly, such applications require relatively high radial expansion forces. Other applications, such as esophageal applications require much smaller forces.
U.S. Pat. No. 4,655,771 issued to Wallsten discloses a stent of the Didcott design particularly adapted for transluminal implantation in blood vessels for treating stenoses or aneurisms. In some applications, such as the esophageal application particularly discussed in the aforementioned patent to Didcott, the stent is temporary. In other applications, such as the blood vessel application discussed in the aforementioned Wallsten patent, the stent is permanent. In permanent installations, the tissue of the body lumen within which the stent is placed tends to grow around the stent such that the stent essentially becomes incorporated with the tissue of the body vessel and thus becomes permanently affixed. However, in the weeks or months before this occurs, the stent is held in position by friction between the outer surface of the stent body and the inner surface of the vessel created by the radial expansion force of the stent. Thus, the resting diameter of the stent, therefore, is selected to be slightly larger than the inner diameter of the vessel so that there is a constant force between the inner wall of the vessel and the outer wall of the stent.
Bioabsorbable stents are also known in the prior art. Bioabsorbable stents are manufactured from materials which, when exposed to body fluids, dissolve over an extended period of time. Thus, such stents are temporary in the sense that they will eventually dissolve and are eliminated from the body. Such stents are permanent, however, in the sense that there is no separate medical procedure needed to remove the stent from the body, it simply dissolves over time. Various bioabsorbable materials that are suitable for stents are known in the prior art including polymers such as poly-L,D-lactide, poly-L-lactide, poly-D-lactide, bioglass, poly(alpha hydroxy acid), polyglycolic acid, polylactic acid, polycaprolactone, polydioxanone, polyglucanate, polylactic acid-polyelethelene oxide copolymers, tyrosine derived polycarbonate, polyglycolide, modified cellulose, collagen, poly(hydroxybutyrate), polyanhydride, polyphosphoester, poly(amino acids) or combinations thereof. Vainionpxc3xa4xc3xa4 at al., Prog Polym. Sci., vol. 14, pp. 697-716 (1989); U.S. Pat. Nos. 4,700,704, 4,653,497, 4,649,921, 4,599,945, 4,532,928, 4,605,730, 4,441,496, and 4,435,590, all of which are incorporated herein by reference, disclose various compounds from which bioabsorbable stents can be fabricated.
Self-expanding braided stents rely on the spring force of the crossing threads that form the stent body as the radially expanding force. The magnitude of the radially expanding force is, therefore, a function of such factors as the number of threads, the size of the individual threads, the flexibility of the individual threads and the crossing angle of the threads. Self-expanding woven stents rely on a separate set of factors including size and number of threads employed, the flexibility of the individual threads, and the particular weave pattern chosen.
These characteristics of the stent, however, must be chosen based on factors in addition to the desired radial expansion force. For instance, the size of the threads is at least partially limited by the size of the lumen within which it will be employed. Further, characteristics of the material forming the stent body and thus the tensile strength and flexibility of the material is limited to materials which can be safely placed in a human body. Stents made from bioabsorbable materials exhibit different properties than corresponding metallic stent designs. Examples of properties that must be controlled when using bioabsorbable materials include degradation rates, material creep, and material position memory.
Accordingly, it is generally desirable to have various means by which to establish the radial expansion force of the stents. This is particularly true with respect to bioabsorbable stents due to the importance of final position memory associated with bioabsorbable materials.
Therefore, it is an object of the present invention to provide methods and designs to achieve optimum performance when employing bioabsorbable materials in stents.
It is another object of the present invention to provide a self-expanding stent with a supplemental mechanism for increasing the radial expansion force.
The invention applies fully to all stent designs, but is particularly adapted to stents that employ bioabsorbable materials, such as molded, braided and woven self-expanding bioabsorbable stents. The preferred embodiment of the invention comprises a radially self-expanding bioabsorbable stent comprised of a tubular body formed from a first plurality of flexible thread elements each extending in a helical configuration around the longitudinal axis of the stent body in a first direction of winding, a second plurality of flexible thread elements extending in a helical configuration around the longitudinal axis of the stent body having the opposite direction of winding, and a separate force applying mechanism associated with the stent body to axially constrict and/or radially expand the stent body.
In accordance with the helical thread structure of the stent body, the stent has a tendency to take a certain diameter and length. In use, the stent is first radially constricted (and axially elongated) in order to allow it to be more easily maneuvered into position in the body vessel. Once in situ, it is released such that it is allowed to radially expand (and axially constrict) back towards its rest diameter and length. Bioabsorbable polymer stents are subject to material memory effects, such that the amount of constrictive force and the degree of constriction can impart a memory to the stent that will alter the extent to which it will return to its original diameter and length.
Typically, the stent""s final rest diameter is chosen to be slightly larger than the inner diameter of the body vessel within which it is placed so that the stent applies a radially outward force against the walls of the vessel tending to hold it in position by friction. The provision of a mechanism for imparting a larger radial diameter memory to the stent, or for supplementing the stent""s radial expansion force increases the frictional force of engagement with the walls without the undesirable side effects of prior art means for increasing the radial expansion force, such as increasing the thickness or tensile strength of the threads, increasing the crossing angle of the threads, or increasing the number of threads.
The inventive mechanisms for increasing the radial expansion force include substituting within the stent body one or more particularly rigid threads relative to the other threads. Another mechanism is forming one or more of the helical threads of two side-by-side threads, one of the threads comprised of standard material and the other comprised of a more rigid material. Even further, one or more bands may be longitudinally attached at their ends to the stent body and their ends. The bands may be elastic or inelastic. The bands would tend to counteract any axially elongating force and/or apply an axially constrictive force. The bands may be attached at their ends to the helically wound threads by adhesive or by mechanical means, such as hooks.
In another embodiment, circular or oval bands are woven into the threads forming the wall of the stent. The bands may be elastic or inelastic. The bands are shaped and positioned to resist axially elongation of the stent.
In some embodiments, the longitudinal bands may be fabricated from a material that shrinks in length when exposed to moisture or body temperature. In other embodiments, the supplemental mechanism is fabricated of a bioabsorbable material such that, after an initial period when the extra radial expansion force is most needed, they dissolve and disappear. A primary benefit of increasing the radial expansion force of a bioabsorbable stent and causing an increase in the diameter of the stent in situ is that this larger diameter imparts memory into the stent. The final diameter achieved with self-expanding bioabsorbable stents is directly related, not only to the radial force generated by the stent design, but also the last diameter achieved immediately following stent delivery.
In accordance with another aspect of the present invention, the radial diameter memory of a bioabsorbable self-expanding stent with or without supplemental radial expanding mechanisms is adjusted by inserting the stent into a body vessel in conjunction with an inflatable balloon wherein inflation of the balloon when the stent is in the desired final position will impart a specific radial diameter memory to the stent by producing a temporary radial diameter greater than the stent body alone could create through its own self-expansion force. After some period of time, the balloon is deflated and withdrawn from the vessel.